Technical Field
Exemplary aspects of the present invention relate to a fixing device and an image forming apparatus, and more particularly, to a fixing device for fixing an image on a recording medium and an image forming apparatus incorporating the fixing device.
Description of the Background
Related-art image forming apparatuses, such as copiers, facsimile machines, printers, or multifunction printers having two or more of copying, printing, scanning, facsimile, plotter, and other functions, typically form an image on a recording medium according to image data. Thus, for example, a charger uniformly charges a surface of a photoconductor; an optical writer emits a light beam onto the charged surface of the photoconductor to form an electrostatic latent image on the photoconductor according to the image data; a development device supplies toner to the electrostatic latent image formed on the photoconductor to render the electrostatic latent image visible as a toner image; the toner image is directly transferred from the photoconductor onto a recording medium or is indirectly transferred from the photoconductor onto a recording medium via an intermediate transfer belt; finally, a fixing device applies heat and pressure to the recording medium bearing the toner image to fix the toner image on the recording medium, thus forming the image on the recording medium.
Such fixing device may include a fixing roller heated by a heater and a pressure roller pressed against the fixing roller to form a fixing nip therebetween. As a recording medium bearing a toner image is conveyed through the fixing nip, the fixing roller and the pressure roller apply heat and pressure to the recording medium, melting and fixing the toner image on the recording medium.
Instead of the fixing roller, the fixing device may include a fixing belt having a thermal capacity smaller than that of the fixing roller and heated by a heater lamp. Instead of the fixing belt, the fixing device may include a fixing film heated by a ceramic heater.
The fixing belt is requested to be heated quickly to shorten a first print time taken to output the recording medium bearing the fixed toner image upon receipt of a print job. Additionally, as the image forming apparatus conveys an increased amount of recording media at high speed, the fixing belt is requested to overcome shortage of heat.
On the other hand, since the fixing film is heated by the ceramic heater situated at the fixing nip, the fixing film is heated insufficiently at an entry to the fixing nip, resulting in faulty fixing. Accordingly, the fixing film is requested to overcome shortage of heat at the entry to the fixing nip.
To address those requests, the fixing device may include a metal thermal conductor as shown in FIG. 1. FIG. 1 is a vertical sectional view of a fixing device 20R1 incorporating a tubular, metal thermal conductor 200 disposed inside an endless belt 101. A heater 300 is disposed inside the metal thermal conductor 200. A pressure roller 400 is pressed against the metal thermal conductor 200 via the endless belt 101 to form a fixing nip N between the pressure roller 400 and the endless belt 101. As the pressure roller 400 rotates clockwise in FIG. 1, the endless belt 101 rotates counterclockwise in FIG. 1 in accordance with rotation of the pressure roller 400, thus conveying a recording medium P bearing a toner image in a recording medium conveyance direction D1. The metal thermal conductor 200 guides the endless belt 101 sliding thereover. The heater 300 heats the metal thermal conductor 200 which in turn heats the endless belt 101, thus heating the endless belt 101 entirely. Since the tubular, metal thermal conductor 200 is disposed opposite the endless belt 101 throughout the entire circumferential span of the endless belt 101, the metal thermal conductor 200 heats the endless belt 101 quickly, thus shortening the first print time and overcoming shortage of heat.
In order to shorten the first print time and save energy further, the endless belt 101 heated by the heater 300 directly, not through the metal thermal conductor 200, is proposed as shown in FIG. 2. FIG. 2 is a vertical sectional view of a fixing device 20R2 incorporating the endless belt 101 heated by the heater 300 directly. As shown in FIG. 2, instead of the metal thermal conductor 200 depicted in FIG. 1, a nip formation plate 500 disposed inside the endless belt 101 presses against the pressure roller 400 via the endless belt 101 to form the fixing nip N between the endless belt 101 and the pressure roller 400. Since the heater 300 heats the endless belt 101 directly at a position other than the fixing nip N, the heater 300 heats the endless belt 101 effectively, shortening the first print time at reduced manufacturing costs while saving energy. A stainless steel support 600 supports the nip formation plate 500 to enhance mechanical strength of the nip formation plate 500 against pressure from the pressure roller 400.
In addition to the heater 300 heating the endless belt 101 directly as shown in FIG. 2, a reflector may be situated inside the endless belt 101 to reflect light irradiating the reflector toward the endless belt 101, thus heating the endless belt 101 effectively and quickly.
In order to downsize the fixing device and decrease the thermal capacity of the fixing device, the endless belt 101 may have a decreased loop diameter. The decreased loop diameter of the endless belt 101 causes the reflector to be disposed opposite the heater 300 with a decreased interval therebetween. Hence, the reflector is susceptible to heat from the heater 300. As the temperature of the entire reflector increases, the temperature of a reflection face of the reflector increases, resulting in tarnishing of the reflection face of the reflector due to thermal oxidation of a material treated with deposition on the reflection face.
Alternatively, the fixing device may include a plurality of heaters: a center heater including a filament that heats a center of the fixing belt in an axial direction thereof and a lateral end heater including a filament that heats each lateral end of the fixing belt in the axial direction thereof. The center heater and the lateral end heater are turned on and off according to the size of the recording medium, preventing overheating of each lateral end of the fixing belt in the axial direction thereof where the recording medium is not conveyed. Additionally, the center heater and the lateral end heater are turned on and off based on the temperature of the center and the lateral end of the fixing belt in the axial direction thereof that is detected by a plurality of sensors disposed opposite the center and the lateral end of the fixing belt.
However, the center heater and the lateral end heater may generate heat unnecessarily at a section where heating is not required, for example, a connection portion connecting the filament of the center heater and the filament of the lateral end heater. Accordingly, heat may be conducted from the connection portion to the reflector, overheating the reflector.